MRAM is a nonvolatile memory that is promising from the viewpoint of a high integration and a high speed operation. In the MRAM, a magnetoresistive element indicating the magnetoresistive effect such as the TMR (Tunnel MagnetoResistance) effect is used. In the magnetoresistive element, for example, MTJ (Magnetic Tunnel Junction) in which a tunnel barrier layer is sandwiched between two ferromagnetic layers is formed. The two ferromagnetic layers are provided with: a magnetization pinned layer (pinned layer) in which a magnetization direction is pinned; and a magnetization free layer (free layer) in which a magnetization direction can be switched.
A resistance value (R+ΔR) of the MTJ when the magnetization directions of the pinned layer and the free layer are anti-parallel becomes higher than a resistance value (R) of the MTJ when the magnetization directions of the pinned layer and the free layer are parallel, because of the magnetoresistive effect. In the MRAM, the magnetoresistive element having this MTJ is used as a memory cell. Then, the memory cell stores a data in a nonvolatile manner by using variations in the resistance value of the MTJ. For example, an anti-parallel state is correlated to a data “1”, and a parallel state is correlated to a data “0”. Writing of the data to the memory cell is carried out by switching the magnetization direction of the free layer.
As a method of writing a data to the MRAM, the asteroid method and the toggle method are known. According to those writing methods, a switching magnetic field required to switch the magnetization of the free layer becomes great approximately inversely proportional to a memory cell size. In short, there is a tendency that a writing current is increased as a memory cell is miniaturized.
As a writing method that can suppress the increase in the writing current in association with the miniaturization of the memory cell, the spin transfer method is proposed (for example, Japanese Patent Publication No. JP-P 2005-93488A corresponding to U.S. Pat. No. 7,193,284). According to the spin transfer method, a spin-polarized current is injected to a ferromagnetic conductor. Consequently, the magnetization is switched by a direct interaction between the spin of conducting electrons of the current and the magnetic moment of the conductor (hereafter, referred to as a spin transfer magnetization switching).
U.S. Pat. No. 6,834,005B discloses a magnetic shift register that uses the spin transfer method. This magnetic shift register uses domain walls in a magnetic body and stores a data. In the magnetic body that is separated into many regions (magnetic domains) through the use of constrictions and the like, a current is injected to pass through the domain walls, and the domain walls are moved by the current. The direction of the magnetization of each region is treated as a record data. The foregoing magnetic shift register is used to record, for example, a large quantity of serial data.
The domain wall motion MRAM that uses a domain wall motion through the use of the foregoing spin transfer is described in Japanese Patent Publication No. JP-P 2005-191032A and WO2005/069368 (corresponding to US Application Publication 2008137405).
The MRAM described in JP-P 2005-191032A includes: a magnetization pinned layer in which a magnetization is fixed; a tunnel insulation layer which is laminated on the magnetization pinned layer; and a magnetization recording layer which is laminated on the tunnel insulation layer. The magnetization recording layer includes a portion in which the magnetization direction can be switched and a portion in which the magnetization direction is not substantially changed. Thus, it is referred to as the magnetization recording layer and not referred to as the magnetization free layer. FIG. 1 is a schematic view showing a structure of the magnetization recording layer in JP-P 2005-191032A. In FIG. 1, a magnetization recording layer 100 has a straight shape. The magnetization recording layer 100 includes a junction portion 103, constriction portions 104 and pairs of magnetization pinned portions 101 and 102. The junction portion 103 overlaps with a tunnel insulation layer (not shown) and a magnetization pinned layer (not shown). The constriction portions 104 are adjacent to both ends of the junction portion 103. The pair of magnetization pinned portions 101 and 102 is formed adjacently to the constriction portions 104. The pinned magnetizations opposite to each other are applied to the pair of magnetization pinned portions 101 and 102. Moreover, the MRAM includes a pair of writing terminals 105 and 106 electrically connected to the pair of magnetization pinned portions 101 and 102, respectively. Through these writing terminals 105 and 106, a current flows, which penetrates through the junction portion 103, the pair of constriction portions 104 and the pair of magnetization pinned portions 101 and 102 in the magnetization recording layer 100. The constriction portion 104 acts as a pin potential for the domain wall. A data is held on the basis of: whether the domain wall exists in the right constriction portion 104 or the left constriction portion 104; or the magnetization direction of the junction portion 103. The movement of the domain wall is controlled by the above current.
The MRAM described in WO2005/069368 uses a step as a means for forming a pin potential. FIG. 2 is a schematic view showing a structure of the magnetization recording layer in WO2005/069368. In FIG. 2, the magnetization recording layer 100 is provided with three regions that are different from each other in thickness. Specifically, the magnetization recording layer 100 is provided with the thickest first magnetization pinned layer 101, the second thickest second magnetization pinned layer 102, and the thinnest junction portion 103 arranged between them. Here, the reason why the thicknesses of the first magnetization pinned layer 101 and the second magnetization pinned layer 102 are different is that the pinned magnetizations opposite to each other are applied in an initializing process. Incidentally, in WO2005/069368, a magnetic semiconductor having an anisotropy vertical to a film surface is used as the magnetization recording layer, and the current for the sake of the domain wall motion is small such as 0.35 mA. Although the tunnel insulation layer and the magnetization pinned layer are arranged on the junction portion 103, they are omitted in FIG. 2. In FIG. 2, the steps at the boundaries between the junction portion 103 and the magnetization pinned layer 101 and between the junction portion 103 and the magnetization pinned layer 102 function as the pin potentials. For this reason, for example, a domain wall 112 remains at the boundary between the junction portion 103 and the magnetization pinned layer 101.
In this way, the domain wall motion MRAM disclosed in the above documents is required to be designed such that the constrictions, the steps and the like are used to generate the pin potentials, and the domain wall constrained therein is moved through the use of the current.
On the other hand, Japanese Patent Publication No. JP-P 2008-34808A (corresponding to US Application Publication No. 2008025060) discloses a method of controlling a domain wall position without any constriction. FIG. 3 is a schematic view showing a magnetization structure of a magnetic storage in JP-P 2008-34808A. In a magnetic wire 140, a plurality of magnetic domains 130 is formed through the use of a plurality of domain walls 135 along its longitudinal direction. Any constriction or any step is not arranged in the magnetic wire. The movement of the domain wall is carried out by a magnetic field or a current pulse, and a domain wall motion distance is controlled on the basis of the width of a pulse. FIG. 4 is a graph showing a relation between a pulse application time (horizontal axis) and a domain wall position (vertical axis) calculated by a simulation. As shown in the curve A, the domain wall has a tendency in which a movement speed becomes 0 and the domain wall stops at a particular time. In JP-P2008-34808A, the pulse application time is set to this stop time, and the domain wall position is controlled.
As the related technique, Japanese Patent Publication No. JP-P 2006-73930A discloses a method of changing a magnetization state of a magnetoresistive effect element that uses a domain wall motion, and a magnetic memory element and a solid magnetic memory that uses the method. This magnetic memory element is the magnetic memory element that includes a first magnetic layer, a middle layer and a second magnetic layer and records a data by using the magnetization directions of the first magnetic layer and the second magnetic layer. The magnetic memory element steadily forms the magnetic domains that exhibit the magnetizations anti-parallel to each other inside at least one magnetic layer and the domain wall that separates these magnetic domains. Then, the magnetic memory element moves the domain wall inside the magnetic layer, and consequently controls the positions of the magnetic domains adjacent to each other to record the data.
Also, as the related art, Japanese Patent Publication No. JP-P 2006-270069A discloses a magnetoresistive effect element and a high speed magnetic recording apparatus, which are based the domain wall motion through the use of a pulse current. This magnetoresistive effect element has a first magnetization pinned layer/a magnetization free layer/a second magnetization pinned layer. This magnetoresistive effect element includes a mechanism for inducing a domain wall generation in a transition region between the magnetization pinned layer and the magnetization free layer, which serves as at least one boundary between the first magnetization pinned layer/the magnetization free layer or between the magnetization free layer/the second magnetization pinned layer. Then, the magnetization directions of these magnetization pinned layers are set approximately anti-parallel, and the domain wall exists in any one of the transition regions of the magnetization pinned layer/the magnetization free layer. In this structure, when the current of a predetermined pulse width is supplied, at the current that does not exceed a direct current density of 106 A/cm2, the domain wall moves between the two transition regions. Hence, the magnetization of the magnetization free layer is switched, thereby detecting the magnetic resistance associated with the change in the direction of a relative magnetization.